In a switch such as a relay, and the like, various types of contacts are used. Among those contacts, a double-break (or a double-make) contact structure is extensively used. As used herein, the “double-break (or double-make) contact structure” means a contact structure which has a function wherein a contact provided on a section functioning as a movable terminal of one movable member (i.e. a movable contact) mechanically contacts with a contact provided on a section functioning as a fixed terminal (i.e. a fixed contact) in each of two fixed members to form two electric connections (i.e. a double-make part), thereby closing an electric circuit between the movable member and the fixed members; and also has a function wherein, from the above mentioned contacting state of the contacts, the contact of the movable member is separated from the contacts of the fixed members to form two electric non-connections (i.e. double-break parts), thereby opening the electric circuit between the movable member and the fixed members. That is, a contact structure in which there are two contact pairs which can take reversibly the contacting state or the non-contacting state is called as the double-break (or double-make) contact structure.
It is noted that such contacts as mentioned above may be in any suitable forms provided on sections which function as terminals of the movable member and the fixed member, usually on their end portions. For example, they may be in various forms of protruding portions, flat plates or portions of such flat plates. In one embodiment, the contact provided on the fixed member is a protrusion, for example, a portion of a sphere (for example, a hemispherical protrusion), and the contact provided on the movable member is a flat plate. In other embodiment, the contact provided on the fixed member may be a flat plate, and the contact provided on the movable member may be a protrusion. It is noted that when the contact is a flat plate, it is possible that no separate plate is provided, but the terminal itself has a function as a contact. It is noted that the contact provided on the movable member may be in a single form which is integrally connected to the movable member by means of an electrically conductive material, and in particular, the contact may be in the form of an elongated and flat strip provided on the movable member. Furthermore, the strip may be provided with a protrusion(s).
In such “double-break (or double-make) contact structure”, a length of its air-gap is doubled in comparison with that of the air-gap of a single-break contact structure having one contact pair. Therefore, such contact structure is said to have an advantage of causing less fusion likelihood between the contacts because energy of arc which is generated when a circuit is opened is dispersed into two air-gaps. Thus, such contact structure is extensively used.
For example, WO 2009/128535 proposes a circuit protection device which uses the above mentioned “double-break (or double-make) contact structure”. This device comprises a circuit switching member having a bimetal component as a circuit switching element and a movable contact as well as a PTC member. This circuit protection device is constructed such that, in a state wherein each of two movable contacts and each of two fixed contacts are in contact respectively (i.e. the double-make state) and a current is flowing through a circuit, when an excess current flows through the circuit, for example, due to occurrence of some abnormal state and a temperature of the bimetal component exceeds its threshold temperature, a state wherein the movable contacts are displaced to be separated from the fixed contacts (i.e. a double-break state) by the deformation of the bimetal component, thereby opening the circuit to protect the circuit and/or an electrical apparatus incorporated into the circuit. Then, when the abnormal state is resolved, the temperature of the bimetal component is decreased to return to its original shape, so that the movable contacts and the fixed contacts are in contact at two positions to flow the current through the circuit again.
In order that such a circuit protection device adequately provides a circuit protection function, it is necessary to allow contact and separation between the contacts to sufficiently and surely arise at two positions by the deformation of the bimetal component.
International Publication No. WO 2009/128535 is a prior patent reference.